1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens suitable for an image-taking system such as an electronic camera (such as a video camera, or a digital still camera, etc.), a film camera, a broadcasting camera, etc.
2. Description of the Related Art
In zoom lenses used for image-taking systems (image-taking apparatuses) such as television cameras, video cameras, etc., there is an increasing demand for a large aperture ratio, a large zoom ratio, and a wide field angle, in addition to being a small-size lens system as a whole.
Among the zoom lenses, four lens units including, in order from an object side to an image side, a first lens unit (focus lens unit) having a positive refractive power for focusing, a second lens unit (zoom lens unit) having a negative refractive power for zooming, a third lens unit (compensation lens unit) having a positive or negative refractive power for correcting for the variation of image surface during zooming, an aperture stop, and a fourth lens unit (relay lens unit) having a positive refractive power for forming an image is disclosed, for example, in Patent Documents 1 to 5.
A front focusing type arranged focus lens unit closer to the object side than the zoom lens unit is more advantageous for a manual focusing type, since the amount of movement of the focus lens unit does not vary even during zooming when an object distance is constant. For this reason, the front focusing types are widely employed in broadcasting zoom lens or in commercially used zoom lens which requires many manual operations.
A rear focusing type arranged focus lens unit closer to the image side than the zoom lens unit is widely employed in auto focusing zoom lens type, since it is advantageous for decreasing the size and weight of the focus lens unit.
In view of this point, in the zoom lens which comprises four lens units having positive, negative, negative, and positive refractive powers, there is known a zoom lens in which a lens unit closer to the image side than the zoom lens unit is moved during auto focusing operation and a lens unit closer to the object side than the zoom lens unit is moved during manual focusing operation. (For example, see Patent Document 6.)
In addition, as a zoom lens including five lens units, that is, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, there is known a rear focusing type zoom lens in which the zooming is performed from the wide-angle end to the telephoto end by moving the second lens unit to the image side. Also, the variation of image surface due to the zooming is corrected by moving the fourth lens unit, and the focusing operation is performed by moving the fourth lens unit. (For example, see Patent Document 7.)
Furthermore, as a zoom lens including five lens units, that is, in order from the object side to the image side, a first lens unit having a positive refractive power and being fixed during zooming, a second lens unit, used for zooming, having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power and correcting the variation of image surface caused by the zooming, there is known a rear focusing type zoom lens in which at least one of the third lens unit and the fourth lens unit is moved during the zooming and the fifth lens unit is moved during focusing operation. (For example, see Patent Document 8.)
Patent Document 1: Japanese Patent Application Laid-open No. H2-118510.
Patent Document 2: Japanese Patent Application Laid-open No. H2-208618.
Patent Document 3: Japanese Patent Application Laid-open No. H3-123310.
Patent Document 4: Japanese Patent Application Laid-open No. H3-145615.
Patent Document 5: Japanese Patent Application Laid-open No. H4-138407.
Patent Document 6: Japanese Utility Model Registration Publication No. S62-43286.
Patent Document 7: Japanese Patent Application Laid-open No. H8-146295.
Patent Document 8: Japanese Patent Application Laid-open No. H5-215967.
In the zoom lens disclosed in Patent Documents 1 to 5, the fourth lens unit has one negative lens element and two positive lens elements, in order from the object side to the image side. However, since the incident reduced tilt angle α4 of the axial ray to the fourth lens unit is large, variation of an incident height h4 of the axial ray becomes large during focusing operation with the fourth lens unit, such that variations of axial aberrations such as spherical aberration, longitudinal chromatic aberration, etc., tend to increase.
In addition, in the zoom lens disclosed in Patent Document 6, the fourth lens unit has one negative lens element and two positive lens elements, in order from the object side to the image side. However, since an Abbe's number ν4n of a material of the negative lens element and Abbe's numbers ν4p of materials of the positive lens elements are close each other, the curvatures of the respective lens elements in the fourth lens unit are increased, thereby increasing the off-axial aberration during focusing operation. Also, because the weight of the fourth lens unit is increased and because a larger driving power for focusing operation is needed, the power consumption or the size of a driving mechanism tends to be increased.
In addition, since the refractive power of the fourth lens unit is repressed and small, a new lens unit having a positive refractive power for converging the light flux diverging from the third lens unit must be arranged closer to the object side than the aperture stop. Further, in order to alleviate the degree of divergence of the light flux from the third lens unit, the refractive power of the third lens unit must be reduced. As a result, the amount of movement of the third lens unit during zooming tends to be increased, such that the total length of the zoom lens tends to be increased.
Further, a zoom lens of the rear focusing type is also disclosed, for example, in Patent Documents 9 to 13.
Patent Document 9: Japanese Patent Application Laid-open No. 2001-21803.
Patent Document 10: Japanese Patent Application Laid-open No. 2001-343583.
Patent Document 11: Japanese Patent Application Laid-open No. H11-305124
Patent Document 12: Japanese Patent Application Laid-open No. 2001-194586.
Patent Document 13: Japanese Patent No. 2561637.
At a zoom position with a focal length f of a zoom lens unit, let us define an image-forming point as being i0 when an object is at infinity and the image-forming point as being i(obj) when an object distance is obj. Then, the variation amount Δsk=i(obj)−i0 of the image-forming point when the object distance is varied is expressed by the following expression from a Newton's formula:Δsk=f2/(obj−f)  (1)Therefore, when the focusing operation is performed with a partial system B1 closer to the object side than the zoom lens unit, the variation amount ΔskB1 of the image-forming point at the object distance obj is expressed by the following expression:ΔskB1=fB12/(obj−fB1)  (2)and it is constant regardless of the zoom position (variation of magnification). Therefore, a movement amount ΔxFF during focusing operation of a partial system (focus lens unit) FF of the partial system B1 is not varied regardless of a zoom state.
However, when the focusing operation is performed with a lens unit closer to the image side than the zoom lens unit, the displacement ΔskB1 of the image-forming point in the partial system B1 is increased or decreased by a partial system B2 which is a zoom lens unit, and as a result, the displacement ΔskB2 can be approximately expressed by the following expression:ΔskB2={fB12/(obj−fB1)}·βB22  (3)Therefore, when the focusing operation is performed with a partial system FR in the partial system B3 which is the lens unit closer to the image side than the zoom lens unit, the movement amount ΔxFR of the partial system FR is increased proportionally to the square of the image-forming magnification βB2 of the zoom lens unit. Therefore, as the zoom ratio is increased, the movement amount of the partial system FR during focusing operation is increased and the space for moving the partial system FR is increased. Thus the driving power for moving the partial system FR by a desired amount is also increased, thereby increasing the overall size of the zoom lens.
The zoom lens disclosed in Patent Document 12 described above is comprised four lens units including lens units having positive, negative, positive, and positive refractive powers, in order from the object side to the image side, and since the fourth lens unit is shared for use as a compensator (for correcting for the movement of image surface due to the zooming) and for use of focusing operation, there is a problem in that the zoom operation cannot be performed manually.
The zoom lens disclosed in Patent Document 6 is a zoom lens including four lens units having positive, negative, negative, and positive refractive powers, in order from the object side to the image side, and the first lens unit is used for the manual focusing operation, the second lens unit is used as a variator (zoom lens unit), the third lens unit is used as a compensator (image surface-movement correcting unit), and the lens unit closer to the image side than the fourth lens unit is used as the focus lens unit for auto focusing operation.
The zoom lens disclosed in Patent Document 6 has a relatively small zoom ratio of 6 times or less, and the focal length at the zoom position of the telephoto end is relatively short, more or less 70 mm. For this reason, if the zoom ratio is further increased and if the focal length at the telephoto end is increased, the movement amount of the lens unit closer to the image side than the fourth lens unit is rapidly increased during focusing operation, so that there is a problem in that the whole zoom lens is large-sized.